Blow-molding is a notoriously well known process for forming hollow articles, typically containers for packaging a variety of commercial products in solid or liquid form. Conventional blow-molding techniques (e.g., extrusion or injection blow-molding methods) use a pressurized fluid (typically air or an inert gas such as nitrogen) to expand a hot parison or preform against a female mold cavity. In such a manner, the expanded parison or preform will conform to the shape and configuration of the female mold cavity. Upon cooling of the expanded parison or preform, the mold cavity may be parted and the blow-molded article removed.
Conventional blow-molding methods and apparatus, however, are limited in the type of hollow articles that can be produced. For example, conventional blow-molding methods and apparatus are typically limited to forming hollow articles having relatively small diameter openings (typically no greater than the nominal diameter of the open end of the parison or preform from which the article is made). That is, the physical dimension of the blow pin (which introduces pressurized fluid (e.g., air) into the parison or preform) is typically minimized since it must be of a size that physically penetrates the open end of the parison or preform, and since the mold cavity must be sealed about the its periphery (so as to prevent the escape of pressurized fluid). As a result, the largest effective size of the opening formed in conventional blow-molded articles (i.e., which usually corresponds to the open end of the parison or preform through which the blow pin penetrates) is typically less than the nominal diameter of the parison or preform.
As can be appreciated, there are a number of hollow articles, for example packaging containers for foodstuffs, that have relatively large-sized and/or irregular shaped (non-circular) openings which could be candidates for manufacture by blow-molding processes if a novel blow-molding technique was provided that would allow such large-sized and/or irregular shaped openings to be formed in situ. It is towards providing such novel blow-molding processes and apparatus for performing the same that the present invention is directed.
According to the present invention, novel methods and apparatus are provided whereby a generally tubular body of plastics material having a nominal cross-sectional diameter D.sub.1 and an open end which is dimensionally expanded to a significantly greater cross-sectional dimension D.sub.2. A blow pin assembly having a desired peripheral surface that corresponds to the dimension and shape of the opening to be formed in the resulting blow-molded article may then be brought into operative association with the dimensionally expanded open end of the tubular body.
The mold cavities are then moved into enveloping relationship with the tubular body so that the open end thereof is sealed against the peripheral surface of the blow pin assembly. Pressurized fluid is then injected into the interior of the tubular body to urge it into conformance with the mold cavity walls while maintaining the open end thereof in its dimensionally expanded state. When cooled, the mold cavities are parted and the blow-molded article removed. In such a manner, blow-molded articles having significantly larger-sized and/or irregularly shaped openings (i.e., corresponding to the dimension and configuration of the peripheral surface of the blow pin assembly) are provided.
In preferred embodiments, the open end of the tubular body is dimensionally expanded using a novel biaxial stretching assembly. In this regard, the stretching assembly of this invention includes a pair of housing blocks moveable between lesser and greater spaced positions relative to one another along a first axial direction. Each of the housing blocks includes a split pin subassembly which includes first and second pin portions. These pin portions are moveable towards and away from one another along a second axial direction (different from, and preferably normal to, the first axial direction) between an initial position (wherein the pin portions are closely adjacent one another) and a final position (wherein the pin portions are separated from one another). The split pin subassemblies are, moreover, movable between an advanced position (wherein the pins extend beyond the housing blocks) and a retracted position (wherein the pins are housed within the housing blocks).
The stretching assembly is then brought into operative position with the open end of the tubular body such that the housing blocks are in their lesser spaced position and with the first and second pin portions in both their initial and extended positions. Thereafter, the housing blocks are moved into their greater spaced position, and the pin portions of each split pin subassembly are moved into their final position so that the open end of the tubular body is biaxially stretched along the first and second axial directions and is thereby dimensionally expanded (as compared to the nominal diameter of the tubular body).
As described previously, the blow pin assembly may then be moved into the dimensionally expanded tubular body open end. Thus, when the pin portions of the split pin subassemblies are retracted, the tubular body will be draped about the relatively large-sized peripheral surface of the blow pin assembly. The open end of the tubular body is thereby positioned so as to be fluid-sealed against the peripheral surface of the blow pin assembly when the mold cavities are brought into enveloping relationship with the tubular body.
Further aspects and advantages of the present invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments therof.